Process of treating residual positive silver halide images with organic sulfur to render said images oleophilic



United States Patent 3,099,209 PROCESS OF TREATING RESIDUAL POSITIVESIL- VER I-IALIDE IMAGES WITH ORGANIC SULFUR T0 RENDER SAID IMAGESOLEOPHILIC Rudolph E. Damschroder and Edward C. Yackel, Rochester, N.Y.,assignors to Eastman Kodak Company, Rochester, N.Y., a corporation ofNew Jersey No Drawing. Filed Feb. 26, 1960, Ser. No. 11,158 7 Claims.(Cl. 101-1492) This invention relates to a lithographic printing processand more particularly to a photographic method for the preparation oflithographic printing plates.

In lithography a greasy ink is employed and the printing plate comprisesa support coated with a colloid carrier which when wet is hydrophilicand carries a line or dot image which is oleophilic.

The oleophilic properties of the image and hydrophilic properties of thesurrounding support surface are relative in nature to some extent. Whenusing an image bearing plate for printing, both ink and water areapplied and the colloid coated on the support is sufiicientlyhydrophilic to accept water but not the greasy ink, while the imageaccepts the ink but not the water. Hence, to obtain good prints it isnecessary that the difference in hydrophilic and oleophilic propertiesof the image and the back ground surface is sufficiently great that whenwater and ink are applied, the image will accept suflicient ink withoutthe background accepting any ink at all. Hence, lithographic printingplates are supports the nature of whose surface is such that when wettedwith water will not accept greasy ink.

The practice in making a photolithographic printing plate is to producean oleophilic image on the hydrophilic support which can be put into aninking machine whereby large numbers of copies can usually be made veryrapidly from the one plate.

' Where only a few hundreds of copies are required, or even up to abouta thousand copies, it is quite common to use a paper type printingplate, but where many thousands of copies are required it is necessaryto use a more substantial type of printing plate and the ones commonlyused are made of aluminum sheet or zinc sheet. From these metal sheetsit is a common practice in the art to take as many as twenty thousandcopies. Aluminum sheet employed in the art is provided with an inerthydrophilic surface. This is often done by treating the aluminum toproduce thereon an aluminum compound which is hydrophilic for instance,aluminum oxide as in the case of anodised aluminum, aluminum silicateand chromati zed aluminum.

When making such printing plates, usually an ordinary negative is firstmade and then this is used for making a positive oleophilic image on theprinting plate. A large number of processes have been used and suggestedfor doing this.

A printing process employing greasy ink on metal plates was suggestedmany years ago by K. W. C. Webb in which an image formed of silverhalide or silver and mercury was brought into contact with a smoothmetal surface such as a polished copper plate in the presence of certaindeposition agents. The plates used in that suggested process, forinstance the polished copper plate, were not as such suitable aslithographic printing plates and it was necessary to etch the barecopper parts after the image had been formed. One object of the presentinvention is to provide a process which entirely avoids etching afterformation of the image by making the image areas very oleophiliccompared with the background areas.

In view of the wide variety of commercial uses of 3,099,209 PatentedJuly 30, 1 963 "ice photolithographic copying, it is very desirable tohave a process which is both simple and efiective.

'An object of the present invention is to provide a process whichsatisfies these requirements in a very surprisingly simple way by merelyforming a negative silver image in a silver halide emulsion layer andtreating the residual positive silver halide image in the presence ofthe silver negative image with solutions of certain organic sulfurcompounds which react with the silver halide positive image in theemulsion layer to render it oleophilic whereas the hydrophilic propertyof the silver negative image in the emulsion layer is not appreciablyaffected. This treatment of the silver'halide positive image areas ofthe emulsion with the solutions of the sulfur compounds can very easilybe done by swabbing the emulsion layer with the solution, bathing theemulsion layer in the solution for a short time or otherwise applyingthe solution to the emulsion layer in any convenient manner to renderthe residual silver halide oleophilic and the emulsion surface thusink-receptive in that region.

The organic sulfur compounds which can be used as described alone or incombination are the following thiols Lmercaptobenzothiazole The abovethiol compounds are further characterized by forming with the silverhalide, an oleophilic silver compound more insoluble in water than thesilver halide used, thus the thiols will not wash out of the emulsionlayer if a washing step is used in the process or during the normaltreatment of the oleophilic image with water and greasy ink. v

Adducts and unsaturated derivatives, e.g. cyanamid, chalcone ordisulfide derivatives of the above thiol compounds which hydrolyze inalkaline solution to yield the above thiol compounds and no otherphotographically active species act as donors, in situ, of the abovethiols. These thiol donors can be present either directly in theemulsion layer or in a layer contiguous to it. The thiol donors can alsobe present in the emulsion together with an alkaline developercomposition so that when the element is exposed and then subjected tomoisture or heat, or both, in the presence of the alkali, the developercomposition develops the latent silver negative image to a visible imagewhile the thiol donor simultaneously splits to release a thiol of ourinvention which reacts with the residual silver halide positive image torender it oleophilic and capable ofacting as alithographic plate.

It is interesting to note that the compound S-nitroindazole can be usedin a similar manner to obtain an elecphilic image in the region of asilver halide positive image. Many organic sulfur compounds are inactiveor do not react at the surface of emulsion layers in the region of thesilver halide positive image in the presence of a silver negative imageand may react only in the region of the silver image or render theentire emulsion surface more or less oleophilic, e.g. the following arerepresentative of a large number which have been found to be inactive inour invention in that they did not render silver halide oleophilic:

a,o-Dimercapto adipic acid 2,5 -dimercapto 1,3 ,4-thiadiazole B-Mercaptoethyl amine HCl Di ethylaminoethanethiol HClB-Carboxymethylmercaptobenzalacetophenone The following organic sulfurcompounds were found to be unacceptable in that while they imparted somedegree of oleophilictivity to the emulsion surface, they failed todifferentiate between the silver negative image and the silver halidepositive image:

Thiosalicycl-ic acid l-phenyl-S-mercapto tetrazoleZ-mercapto-6-nitrobenzothiazole N-Methyl-2-rnercapto benzamideBiphenyl-4,4'-dithiol t-Dodecylmercaptan benzamide Accordingly, ourinvention contemplates the preparation and use of a lithographicprinting plate prepared by treating a developed but unfixed silverhalide emulsion layer, such as a gelatin-silver halide emulsion layercontaining a silver negative image and residual silver halide positiveimage, with the mentioned compounds alone or in combination so as torender the surface of the emulsion layer oleophilic only in the regionof the silver halide positive image with the result that upon subsequentacidification of the emulsion layer and inking, lithographic prints canbe made in the usual manner.

The fiollowing example will serve to illustrate the process of ourinvention:

A photographic paper having a contact speed gelatinsil'ver chlorideemulsion was exposed to a graphic subject (line or halftone subject),then developed for 30 seconds at 70-75 F. in a conventionalElon-hydroquinone photographic paper developer solution after which itwas removed from the developing bath and squeegeed. The surface was thentreated for 10 to 20 seconds with a percent solution of p-toluenethioldissolved in 5 percent sodium hydroxide solution. The surface thustreated was rinsed, acidified with 5 percent acetic acid solution andplaced on a Multilith printing press. The undeveloped areas of theemulsion were found to be oleophilic and printed a positive image, thep-toluenethiol presumably being reactive with the undeveloped silverhalide to form an oleophiliic surface.

When alkaline solutions of the other effective thiol compounds mentionedwere used in a similar manner for treating a developed silver halideemulsion layer, the surface became oleophilic in the region of theundeveloped silver halide and printed readily in a lithographic printingpress.

In the process of the invention illustrated above, after development ofthe emulsion in the conventional manner, the emulsion may then beacidified with dilute acid solution to stop development and after abrief wash of about 2 minutes in running water, treated with thealkaline solution of the thiol compound. The subsequent acidificationstep may be carried out in a convenient manner, for example, byapplication of dilute acetic acid solution or other acid fountainsolution on the printing press.

The hydrophilic organic colloid of the silver halide emulsion layersused in the invention is not especially critical and may includegelatin, colloidal albumen, zein, partially hydrolyzed cellulose esters,polyvinyl alcohol, partially hydrolyzed polyvinyl ester, polyacrylamide, and other hydrophilic naturally occurring and syntheticorganic colloids. Similarly, the silver halide emulsions may be selectedfrom well-known emulsions containing silver chloride, silver bromide andsilver iodide or mixtures thereof as well as containing optical andchemical sensitizing agents, fog-stabilizing compounds, emulsionhardeners, pl asticizing compounds and wetting agents.

We claim:

. l. A lithographic printing process which comprises reacting theresidual silver halide remaining after formation of a developed silverimage in the emulsion layer with an alkaline solution of a thiolselected from the group consisting of p-toluenethiol, potassium n-butylxanthate, Z-mercapto-S-phenyl-1,3,4-oxadiazole, Z-mercaptobenzothiazole,Z-mercaptobenzimidazole and 2-ethyl-hexanethiol until the surface of themoist emulsion layer in only the region of the undeveloped silver halidehas become receptive to greasy inks, acidifying the emu sion layer,inking the moist emulsion layer and printing a lithographic printtherefrom.

2. The process of claim 1 wherein the emulsion layer is a gelatin-silverhalide emulsion layer.

3. The process of claim 1 wherein the thiol used is ptoluenethiol.

4. The process of claim 1 wherein the thiol used is potassium n-butylxanthate.

5. The process of claim 1 wherein the thiol used isZ-mercapto-S-phenyi-1,3,4-oxadiazole.

6. The process of claim 1 wherein the thiol used isZ-mercaptobenzothiazole.

7. The process of claim 1 wherein the thiol used isZ-mercaptobenzimidazole.

References Cited in the file of this patent UNITED STATES PATENTS2,453,346 Russell Nov. 9, 1948 2,843,491 Allen et a1. July 15, 1958FOREIGN PATENTS 1,058,844 Germany June 4, 1959 1,064,343 Germany Aug.27, 1959 1,064,807 Germany Sept. 3, 1959 758,759 Great Britain Oct. 10,1956

1. A LITHOGRAPHIC PRINTING PROCEESS WHICH COMPRISES REACTING THERESIDUAL SILVER HALIDE REMAINING AFTER FORMATION OF A DEVELOPED SILVERIMAGE IN THE EMULSION LAYER WITH AN ALKALINE SOLUTION OF A THIOLSELECTED FROM THE GROUP CONSISTING OF P-TOLUENETHIOL, POTASSIUM N-BUTYLZANTHATE, 2-MERCAPTO-5-PHENYL-1,3,4,-OXIDIAZOLE, 2-MERCAPTOBENZOTHIAZLE,2-MERCAPTOBENZIMIDAZOLE AND 2-ETHYL-HEZANETHIOL UNTIL THE SURFACE OF THEMOIST EMULSION LAYER IN ONLY THE REGION OF THE UNDEVELOPED SILVER HALIDEHAS BECOME RECEPTIVE TO GREASY INKS, ACIDIFYING THE EMULSION LAYER,INKING THE MOIST EMULSION LAYER AND PRINTING A LITHOGRAPHIC PRINTTHEREFROM.